On a TV program that I watched last night (CBC's Marketplace) they talked about the fee that major car rental companies add to the cost of a car rental if you collect airline Frequent Flyer points. I never think to add the one plan that I'm collecting points on when renting a car, but it was certainly a 'heads up' that should I consider doing this in future, to check to see what the rental agency is going to charge for the points and whether it's worthwhile or not.
Since the practice has been going on for a few years now, it's probably something many people are aware of, but just thought I'd mention it for those who aren't.
For example, this is what Avis charges: www.avis.com/car-rental/content/display.ac…
On the car rental invoice, the charge (listed with all those other mysterious charges) is described as "FTP" (Frequent Traveler Program)